1. Field of the Invention
The present invention relates to an alignment device for a semiconductor exposure apparatus equipped with a projection optical system (projection lens) for transferring a pattern, formed on a recticle (mask), onto a wafer, and more particularly to an alignment device for relative positioning of the reticle and a substrate (wafer) by means of alignment light of a wavelength different from that of the exposure light for transferring the reticle pattern onto the substrate.
2. Related Background Art
Conventional alignment device for aligning the reticle and the wafer by detecting an alignment mark on the wafer through the projection objective lens is so designed to employ alignment light of a wavelength different from that of the exposure light, in order not to affect the photoresist coated on the wafer.
However, such alignment based on the alignment light of the wavelength different from that of the exposure light results in a drawback of chromatic aberration generated in the projection lens, and methods have been proposed, for example in the European Patent No. 393775A and in the U.S. Pat. No. 4,492,459, for correcting such chromatic aberration.
In the EP393775A, the chromatic aberration, resulting from the alignment light of the wavelength different from that of the exposing light, is corrected by positioning a correction lens on the optical axis at the entrance pupil of the projection lens, and the alignment is conducted by detecting .+-.1st-order diffraction light from the wafer mark.
Also in the U.S. Pat. No. 4,492,459, the chromatic aberration, generated when the alignment light passes through the projection lens, is corrected by a correction optical system positioned in or outside the exposing optical path between the reticle and the projection lens, and the alignment is conducted by detecting, through the projection lens, an image of the reticle mark formed on the wafer and the wafer mark.
In the configuration of the above-mentioned EP 393775A, the lens for correcting the chromatic aberration is positioned at the center of the entrance pupil of the projection lens, and is constructed so small as not to undesirably affect the exposing light.
However, this configuration involves a basic drawback that, though the improvement in precision of alignment requires a finer pitch of the diffraction grating constituting the wafer mark, such finer pitch of the diffraction grating expands the distance of the .+-.1st-order diffracted lights to be detected at the entrance pupil of the projection lens so that the correction lens cannot be made smaller in size. Consequently the correction lens becomes inevitably so large as to detrimentally affect the exposing light, and such configuration is unable to comply with the finer alignment.
Also the above-mentioned U.S. Pat. No. 4,492,459 allows, in principle, to correct the longitudinal chromatic aberration of the projection lens, by positioning a correction optical system in or outside the exposing optical path between the reticle and the projection lens.
However, when the image of the wafer mark is observed through the projection lens, because the alignment light is longer in wavelength than the exposing light, said image may enter the exposure area on the reticle due to the chromatic aberration of magnification. In such case, said image of wafer mark can be shifted from the exposure area by correcting the chromatic aberration of magnification with a parallel-faced flat plate of variable angle, positioned outside the exposing optical path between the reticle and the projection lens. However, such method is impractical because said parallel-faced flat plate intercepts a part of the exposing light.
Also in so-called through-the-reticle (TTR) system in which the alignment is conducted through the reticle and the projection lens, or in so-called through-the-lens (TTL) system in which the alignment is conducted through the projection lens, the light emerging from the alignment mark of the substrate (wafer) is taken out by a mirror positioned above or below the reticle, but said mirror may intercept a part of the exposing light so that the arrangement of the alignment optical system becomes difficult if the image of wafer mark tends to shift into the exposure area of the reticle by the chromatic aberration of magnification of the projection lens.
Furthermore, though the projection lens is generally well corrected for the chromatic aberrations (longitudinal chromatic aberration and chromatic aberration of magnification) for the light of exposing wavelength, the designing and manufacture of the projection lens become more difficult if it is to be corrected also for the chromatic aberrations for the alignment light of another wavelength.
Particularly, the projection lens employing an excimer laser as the exposure light source can only employ limited lens materials such as quartz and fluorite, and the adhesion of these material for correction of chromatic aberration is difficult due to the high intensity of the excimer laser. It is therefore difficult to achieve correction also for the chromatic aberration for the alignment light of a wavelength different from that of the exposing light, and the designing and manufacture of such projection lens have therefore been difficult.
The present invention, attained in consideration of the foregoing background, is to provide an alignment device capable of correcting the longitudinal chromatic aberration of the projection lens and simultaneously controlling the chromatic aberration of magnification, despite of a relatively simple configuration, thereby facilitating the positioning of the alignment optical system and rendering the designing and manufacture of the projection lens easier.
The chromatic aberration of magnification in the present invention means the lateral chromatic aberration and defines the aberration between the crossing positions, on the Gaussian image plane, of principal rays respectively of an off-axis light of a wavelength same as that of the exposing light, focused on said Gaussian image plane by passing through the projection lens, and of the alignment light of a wavelength different from that of the exposing light, focused on or in front of or behind said Gaussian image plane by passing through said projection lens. Also the amount T of the chromatic aberration of magnification (or lateral chromatic aberration) is defined by .DELTA.T=.sym..delta.2-.delta.1.vertline., wherein .delta.1 is the distance from the crossing position, on said Gaussian image plane, of the principal ray of the off-axis light of a wavelength same as that of the exposing light, focused on said Gaussian image plane by passing through the projection lens, to the optical axis position of the projection lens on said Gaussian image plane, while .delta.2 is the distance from the crossing position, on said Gaussian image plane, of the principal ray of the alignment light of a wavelength different from that of the exposing light, focused on or in front of or behind said Gaussian image plane by passing through said projection lens, to the optical axis position of the projection lens on said Gaussian image plane.